Marshall MG TDA7293 PCB Assembly Wired Backwards??? Help!

[haunt26]

Hey guys, I was hoping I could get some info from you guys. This is a long post but it's an interesting one.

So my Marshall MG100HDFX head has a blown TDA7293 chip and thanks to the info I found on this forum I was able to find and order a replacement TDA7293 PCB assembly from Antique Electronic Supply. So I installed the replacement in my amp (used thermal compound, replaced fuse, cabled everything correctly to my MG speaker cab etc.) and it blew again, but this time a small capacitor on the PCB board exploded. I watched it happen because I had the amp assembly outside of the head when I powered it on.

This is what it looked like after I took it out of my amp:



I then realized that the wiring on the replacement they sent me was backwards and thought maybe this is what caused it to fail. So I did some research on why capacitors explode and found out that its due to backwards wiring because some capacitors are "polarized" and can only be used one way, so when they're wired backwards they can fail and explode.

This is a picture of the stock TDA7293 PCB assembly that came with my amp with the correct wiring of a yellow wire on the far left and a black wire on the far right(I already removed the TDA7293 chip on it BTW):



As you can see in the picture, the wire on the far left is yellow and the wire on the far right is black. This is the correct wiring because my amp worked fine until it blew due to a friend incorrectly cabling it when recording. If you go to the Antique Electronic Supply website, you can see in the picture they have that the PCB assembly is wired like this as well. All of the pictures I have found online of the TDA7293 PCB assembly have this same wiring pattern so I'm pretty confident that this is the correct wiring pattern.

So I contacted Antique Electronic Supply and told them that the part failed because it was wired backwards so they sent me another replacement and...........lo and behold this one was ALSO wired backwards.

Here is a picture of the second replacement part they sent me, pay attention to the wiring and compare to the one above:



As you can see, the wiring is backwards. So my question to you guys is am I right about the wiring? Does it matter if the wiring is backwards? I'm no expert, but I'm pretty sure it does matter because the colors of the wires represent different things like positive and negative charges so when it's wired incorrectly and connected to the mainboard of my amp it will blow, right? Hopefully some of you with more knowledge would be able to give me more info.

I'm not going to install this second replacement they sent me because I'm pretty sure it will blow again. Let me know what you guys think!

[Enzo]

We will see your pictures in the mroning, I assume, but for now, I generally just replace the IC rather than the whole little module. Of course if you popped the caps on the module it needs further repair.

[haunt26]

I took some more pictures that better show what I'm talking about.

So this is the stock TDA7293 PCB that was in my amp from the factory:



You could see in the picture how the wiring pattern is the same going from the small PCB to the mainboard of the amp. This is what I believe is the proper configuration for the wiring.


Now take a look at the replacement they sent me:



You can see that the wires on the small TDA7293 PCB are reversed and so the they have to cross over in order to plug into the mainboard properly.

This is what the problem is right?

[Enzo]

SO it appears. Either they are made wrong, or they were made for a different model amp.

SO it appears that if the little board itself is right, you have two options. One is to release each wire from the loose conector and reinsert them all in the correct holes. Or just suck the connector off the little board, reverse it and resolder.

[haunt26]

The loose connector is pinned out correctly, it's the connector that's soldered to the small board that's reversed. That was essentially my question, is if the connector being soldered to the small board in reverse could cause it to fail and blow after it's installed and powered on? I'm hoping that's it and not something else that's wrong with the amp.

And like you said, I can remove it and re-solder it in the correct orientation but alas my soldering skills are shit and I would most likely mess it up.

[Jazz P Bass]

Sure looks backwards to me.
If you want to verify it you need to identify which wire goes to the speaker. (on the main board)
Then on the ic board, check that the rear , far right pin (of the TDA7293) goes there.
That is the output pin.
What is really bad, if the wiring is reversed, is that you are mixing up the V+ & V- pins with who knows what.

[soundguruman]

[QUOTE=haunt26;292668]Hey guys, I was hoping I could get some info from you guys. This is a long post but it's an interesting one.

So my Marshall MG100HDFX head has a blown TDA7293 chip and thanks to the info I found on this forum I was able to find and order a replacement TDA7293 PCB assembly from Antique Electronic Supply. So I installed the replacement in my amp (used thermal compound, replaced fuse, cabled everything correctly to my MG speaker cab etc.) and it blew again, but this time a small capacitor on the PCB board exploded. I watched it happen because I had the amp assembly outside of the head when I powered it on.


The new module is destroyed. Replacing the caps won't fix it.
Marshall should cover it under warrantee. I would raise a HUGE HUGE stink.
And, several other parts in the amp are now blown to crap also, 100% certainty, I bet you $1.

BUT: a qualified tech would have realized this, BEFORE they turned on the power.
So, it's 50% YOUR fault. Let's face the facts. Defect board yes, but the parts are only as good as the technician who installs them.
Have you learned your lesson?
What real techs know: Stop assuming, start verifying!

[52 Bill]

Very interesting find. Of course the answer to your question is that the board was damaged by the reversal of the wiring. I never buy the boards, so I would not have seen this before.

I wonder how many of these are out there? If any of you guys out there are still connected to the dealer network, has there been a bulletin on this problem, or do they not know about it yet?

The power supplies are connected into the board at pins 10-15, V- on 10-12 and V+ on 13-15. Pins 1-3 are the signal inputs and ground and pins 4-6 are the standby and power supply grounds. So the reversal would put the full power supply across the inputs of the chip and the chip ground.

And there could be damage to the amp as well, so it will need to be checked as well.

[g1]

Note that the photo on the AES website shows the correct wiring, opposite of what they sent you. If they sent the second one the same way, I would doubt they have heard about the problem yet. So it may be they are not selling many, or this is a new run off the line, wired incorrectly.
There are several other websites selling the P-H216 module, and all are pictured with correct wiring.

[soundguruman]

It's toast. Pass the butter.

[Enzo]

So everyone shows the correct looking wiring, regardless of what comes from their stock.

I note that the MG100HDFX has its 15 pin connector wired different from the 15 pin connector in the Mode 4 (MF350).

[haunt26]

[QUOTE=soundguruman;292897]

The new module is destroyed. Replacing the caps won't fix it.
Marshall should cover it under warrantee. I would raise a HUGE HUGE stink.
And, several other parts in the amp are now blown to crap also, 100% certainty, I bet you $1.

BUT: a qualified tech would have realized this, BEFORE they turned on the power.
So, it's 50% YOUR fault. Let's face the facts. Defect board yes, but the parts are only as good as the technician who installs them.
Have you learned your lesson?
What real techs know: Stop assuming, start verifying!

I bought the amp way back in like 2005, so it's definitely not under warranty anymore. Like you said though, it is my fault since I didn't notice the wires were reversed until after it blew.

How are you so positive that my entire amp is now broken? The only thing I saw blow was the little capacitor on the TDA7293 board. Nothing else from the mainboard was fried or blown. The fuse on the mainboard didn't blow either.

I think I remember reading in another thread about this same TDA7293 issue for this amp that a way to test if the whole amp is broken is to plug headphones into the headphone jack of the amp, turn it on without the installed TDA7293 board, and see if you hear your guitar coming through the headphones? If you do it means that the power amp for the headphones is still good which probably means the whole amp is still good, just need the TDA7293 replaced. If you don't hear sound from the headphones, then it probably means the whole thing is fucked. This sound true to you guys? You guys think this is safe to test if my whole amp is dead or not?

Note that the photo on the AES website shows the correct wiring, opposite of what they sent you. If they sent the second one the same way, I would doubt they have heard about the problem yet. So it may be they are not selling many, or this is a new run off the line, wired incorrectly. There are several other websites selling the P-H216 module, and all are pictured with correct wiring.

I'm going to have to contact AES again and explain to them what's going on and see if they can send me another that is wired correctly. I haven't been able to find the P-H216 module in stock online from any other dealers, not one. AES is the only one who has them in stock, so all of my luck is on them getting me a good one, otherwise I got a dead amp all because of one little part that I can't find a replacement for.

[Enzo]

Oh, guru likes to hear himself make grand statements. Amps are not yes/no equations, they are not either OK or whole dead.

If your headphones work, then everything up to the master volume is functioning. If they don't work, find out why. It is unlikely the reversed power module burnt up the entire circuit.

What is likely to be damaged here might be the standby circuit. Look on the power supply page, bottom center. Your will see transistor TR2 with "STBY" at its collector. IN parallel with that transistor is a cap and a zener. It is conceivable the main V- got send=t down that line and killed those parts. They are probably shorted or they are OK, in most cases.


I(t is hard to say if there are other reverse voltage possibilities, all we can do is find out.

[soundguruman]

Rest in Peace.

[haunt26]

Rest in Peace.

Heh.

Oh, guru likes to hear himself make grand statements. Amps are not yes/no equations, they are not either OK or whole dead.

If your headphones work, then everything up to the master volume is functioning. If they don't work, find out why. It is unlikely the reversed power module burnt up the entire circuit.

What is likely to be damaged here might be the standby circuit. Look on the power supply page, bottom center. Your will see transistor TR2 with "STBY" at its collector. IN parallel with that transistor is a cap and a zener. It is conceivable the main V- got send=t down that line and killed those parts. They are probably shorted or they are OK, in most cases.


I(t is hard to say if there are other reverse voltage possibilities, all we can do is find out.

I found the TR2 transistor on the mainboard, looks fine, doesn't smell like it fried or anything. Couldn't find anything labeled "STBY" and not sure what all that other stuff you said means....

I'll give playing through headphones a try later and see if that at least works.